Chlamydia are human pathogens that cause sterility, blindness, pneumonia and are strongly correlated with the number one cause of death in humans, heart disease. Chlamydia are obligate intracellular bacteria and are perpetuated through a defining biphasic developmental cycle that is intimately linked with pathogenesis. The developmental cycle is governed predominately at the transcriptional level; however, there is a critical deficiency in our understanding of developmental regulatory mechanisms in Chlamydia. The long-term goal in our research is to contribute to a delineation of the key molecular mechanisms that function to regulate chlamydial development and pathogenesis. The research we propose here is designed to define the biological role and essential regulatory mechanisms for the chlamydial transcription factor termed ChxR. ChxR is an atypical member of the OmpR subfamily of response regulators. Our central hypothesis is that ChxR has an important role in regulating middle and late stage gene expression and incorporates a mechanisms similar to, but distinct from, the subfamily of OmpR/PhoB response regulators. Largely due to the current genetic intractability of Chlamydia, the biological role of ChxR is not known. To elucidate the biological role and determine regulatory mechanism of ChxR the following specific aims are proposed: 1) define the direct gene targets of ChxR in vivo, 2) delineate the mechanism integral for chxR transcriptional activation, and 3) determine the intra- and inter- molecular interactions integral to ChxR function. As a result of these studies, we expect to address a fundamental question in the field, 'how do Chlamydia regulate their growth?' Furthermore, OmpR response regulators, like ChxR, are widespread in bacteria and absent in mammals. As such, they are attractive targets for the development of new antimicrobials. Characterization of the molecular mechanisms employed by ChxR for proper function will allow in the future for rationale design of novel molecules that interrupt the function of ChxR and Chlamydia infections.